Detector circuits have been implemented in the prior art with discrete diodes and external impedance matching and video filtering circuitry. At millimeter wave frequencies these diodes are typically single discrete devices that are mounted on a substrate. Impedance matching and video filtering for the detector circuit are also mounted on the substrate, with the input matching network on the input side of the discrete diode, and the output video filter network on the other side of the discrete diode. Such a detector circuit is described in “Development of Compact Broadband Receivers at Submillimeter Wavelengths” by J. L. Hesler, D. W. Porterfield, W. L. Bishop, T. W. Crowe, and P. Racette, 2004 IEEE Aerospace Conference Proceedings IEEE, pages 735-740. The resulting detector circuit is a physically large circuit with limited sensitivity.
Another example of the prior art is a W band detector developed at HRL Laboratories, LLC. This detector circuit is similar to detectors implemented in commercial millimeter wave imaging cameras. The W band detector is implemented using discrete diodes mounted on a circuit board. Input filtering is provided on the input side of the detector, and output filtering on the output side. This W band detector has high sensitivity, but very narrow bandwidth and is described in “Sb-Heterostructure Diode Detector W-band NEP and NEDT Optimization” by H. P. Moyer, R. L. Bowen, J. N. Schulman, D. H. Chow, S. Thomas III, T. Y. Hsu, J. J. Lynch, and K. S. Holabird Proceeding of SPIE Volume 6211, 62110J-1 (2006).
FIG. 1 shows a block diagram of a typical detector circuit. It consists of an input matching network 16 between input transmission lines 12 and 14 and a diode 22, and an output filter network 24 between the diode 22 and the output transmission lines 26 and 28. The diode 22 can be connected in shunt between lines 18 and 20, as shown in FIG. 1 or in series on line 18 (not shown). In either case it is critically important that the output signal between output transmission lines 26 and 28 has a bandwidth extending down to zero Hertz (DC). For example, the output filter network 24 shown in FIG. 1 cannot have a capacitor connected in series between the diode 22 and the output transmission line 26 since this would block zero Hz. Similarly, if the diode is connected in series, then a shunt inductor must be in the circuit in order that the output signal can be read across output transmission lines 26 and 28. This shunt inductor is typically referred to as a “DC return.”
The purpose of the input matching network 16 is to provide an impedance match for maximum delivery of incident power from the input transmission lines 12 and 14 to the diode 22. The output filter network 24 ideally blocks any RF signal frequencies from the output signal, while passing lower frequency video signals to the output transmission lines 26 and 28.
FIG. 2A shows the equivalent circuit of a typical detector diode. It consists of a nonlinear junction resistance 34, a junction capacitance 32, and a series resistance 30. In order to achieve high sensitivity, this device must be impedance matched to an input transmission line. Typical parameter values for an exemplary Backward Tunnel Diode (BTD) are: Cj=8fF, Rj=1400 ohms, and Rs=25 ohms, and the Backward Tunnel Diode is designed to operate up to about 110 GHz. Given these typical parameters, one can show that the diode impedance looks similar to a resistor 36 and a capacitor 38 in series, as shown in FIG. 2B. The values of resistor 36 and capacitor 38 are given approximately by
      R    =                  R        s            ⁡              (                  1          +                      1                                          (                                  ω                  ⁢                                                                                    R                        s                                            ⁢                                              R                        j                                                                              ⁢                                      C                    j                                                  )                            2                                      )              ,      C    =                  C        j            .      To obtain the widest possible bandwidth, which is important for passive millimeter wave imaging applications, communications, and other applications, the diode capacitance can be resonated by an inductance in the input matching network 16. The inductance value chosen will typically be about equal to
      L    =          1                        ω          o          2                ⁢        C              ,where ωo is the center frequency in radians per second of the operational bandwidth of the detector circuit.
The output filter network 24 typically consists of a low pass filter to pass the low frequency video output, while blocking the RF signal from the output signal. For example, for the shunt connected diode of FIG. 1 the output filter network 24 may consist of an inductor in series with the load. This circuit ensures detection down to zero Hz (DC) and has high impedance at the center frequency, thereby blocking the RF signal from the output transmission lines 26 and 28, while not degrading the RF operation of the circuit. An example of a circuit that would not work well for FIG. 1 is a large shunt capacitor connected directly across the diode. Although this allows DC to pass to the output transmission lines 26 and 28, and blocks RF from the output signal, it effectively short circuits the diode at RF, which disrupts the RF operation of the detector circuit.
What is needed is a detector circuit that has a wide operational bandwidth, while isolating the RF input signal from the output signal. The embodiments of the present disclosure answer these and other needs.